In recent years, nickel-metal hydride batteries (secondary battery) using a hydrogen storage alloy for the negative electrode have been drawing attention as an alternative high-capacity alkaline battery to nickel-cadmium batteries.
Not only is the nickel-metal hydride battery already being used as a civil electric source in various small or portable devices, its application to a large battery for use in electric vehicles (EV: Electric Vehicle), hybrid electric vehicles (HEV: Hybrid Electric Vehicle: a vehicle wherein two power sources, i.e. electric motor and internal combustion engine, have been adopted), and the like has become practical.
Examples of main alloy systems being developed as hydrogen storage alloys for use in a negative electrode of a nickel-metal hydride battery include AB5-type hydrogen storage alloys having a CaCu5-type crystal structure, such as LaNi5, and AB2-type hydrogen storage alloys having a Laves phase structure, such as ZrV0.4Ni1.5. In recent years, AB5-type hydrogen storage alloys have become mainstream as batteries in practical use.
Since the electrode cycle life is short with LaNi5 as the AB5-type hydrogen storage alloy, it cannot become a product that endures practical use. The resistance to corrosion in a hydrogen storage alloy using Mm (Misch metal), which is a mixture of rare-earth elements instead of La, and wherein a portion of Ni is further substituted with Co, is known to be improved.
In addition, the composition that is most generally used as the AB5-type hydrogen storage alloy is the hydrogen storage alloy of the Mm—Ni—Co—Mn—Al system, wherein a portion of Ni has been further substituted with Mn and Al (for instance, see Japanese Patent Application Laid Open No. 2001-40442 and Japanese Patent Application Laid Open No. 2001-348636).
However, high power cannot be obtained over a long period with conventionally used AB2-type and AB5-type hydrogen storage alloys, such that they are insufficient, in particular, for application in a large battery for electric vehicles, hybrid electric vehicles, and the like. In addition, they have the problem that cracks occur in the hydrogen storage alloy.